Why is there bubble in sewage treatment? What are the hazards of these bubbles? How to eliminate these bubbles?

Why is there bubble in sewage treatment? What are the hazards of these bubbles? How to eliminate these bubbles?

In the process of sewage treatment, I believe that everyone often encounters a large amount of foam in the biochemical pool, and if it is at rest, it will overflow from the pool, causing serious pollution of the external pool wall of the external equipment, which will deteriorate the operating conditions. Affected the surrounding environment

Type of foam

1.   Initial bubble

• In the initial stage of aeration tank start, the sludge in the aeration tank is not suitable for the water quality of the sewage, and is not suitable for the growth environment, and foam is easily formed. As the sludge adapts to water quality, the foam will decrease.
• At the beginning of the aeration tank start-up, the sludge is relatively small, the sludge load is high, and foam is easily generated. After the amount of sludge increases, the foam will gradually disappear.
• In the initial stage of the operation of the activated sludge process, the surface foam is easily caused by the presence of some surface active substances in the sewage. However, as the activated sludge matures, these surface active substances are biodegraded, and the foam phenomenon gradually disappears.

2.   Denitrifying foam

  When the activated sludge treatment system is operated at low load, denitrification will occur in the sedimentation tank or where the aeration is insufficient to generate nitrogen. The release of nitrogen will reduce the sludge density to some extent and drive some sludge to float. In the case of foaming, the resulting suspended foam is usually not very stable.

3.   Surfactant foam

  The surfactant in the sewage and the surface active substances such as starch, protein, oil and fat all have a molecular structure which contains a polar-nonpolar group, a so-called amphiphilic molecule. Under aeration conditions, one end of the non-polar group protrudes into the bubble, and the polar group is selectively adsorbed by the hydrophilic substance, so that the surface of the hydrophilic substance is converted into a hydrophobic substance and adheres to the bubble water. On the membrane, float up to the surface with the bubbles.

4.   Biological foam

• Foam-related microorganisms mostly contain lipids, which are lighter than water and easily float to the surface.
• The microbes related to foam are mostly filamentous or branched, easy to form a net, can sweep particles and bubbles, and float to the surface. The air bubbles surrounded by the screen increase the tension on the surface, making the bubbles less fragile and the foam more stable.
• The air flotation caused by aeration bubbles is often the main driving force for foam formation. The particles use air bubbles to float, and must be small, light, and hydrophobic. Therefore, when there are oils, lipids and fat-containing microorganisms in the water, surface foaming is likely to occur.

Bubble factor

1. Sludge residence time

  The growth rate of the foam-producing microorganisms is generally low and the growth cycle is long, so long sludge residence time is beneficial to the growth of these microorganisms. Therefore, the activated sludge process using the delayed aeration method is more prone to foaming. In addition, once the foam is formed, the biological residence time of the foam layer is independent of the sludge residence time in the aeration tank, and it is easy to form a stable and durable foam.

2．PH value

  Different filamentous microorganisms have different pH requirements. The growth of amarae is extremely sensitive to pH. The optimum pH is 7.8. When the pH is lowered from 7.0 to 5.0 to 5.6, the foam formation can be effectively reduced. This is mainly because the low pH exceeds the pH limit of the microbial community that produces the foam. Therefore, when the pH is 5.0, the growth can be effectively controlled. However, changes in pH can also cause incompatibility of the activated sludge, resulting in foaming.

3. Dissolved oxygen

  The Nocardia group in the bio-foam is a strictly aerobic microorganism. Under the conditions of anoxic or anaerobic conditions, the matrix growth cannot be used, but it does not die, but the filamentous bacteria are different, and the nitric acid can be utilized. The root acts as the ultimate electron acceptor. Therefore, even in the anoxic section or the anaerobic section of the existing nitrogen and phosphorus removal system, it can be smoothly produced. When the dissolved oxygen is insufficient and the system is operated at a low load, the denitrifying foam is easily generated.

4．Deration mode

  The bubbles generated by different aeration modes are different, and the microbubbles or small bubbles are more favorable for producing biofoam than the large bubbles, and the foam layer is easily concentrated in a region with low aeration intensity.

5．Temperature

  The fungi associated with the formation of biofoam have their own suitable growth temperature and optimum temperature. When the environment or water temperature is favorable for the growth of fungi, foaming may occur. Not only that, temperature also affects the microbial community in the activated sludge system, leading to the production of biofoam, which can be seen seasonally from the production of many biofoams.

Bubble hazard

1. Affect the normal display of the meter

  In particular, sewage treatment plants that use DCS automatic control can cause system misoperation. For the ultrasonic level gauge, it will cause a false liquid level, causing the pump to idling in a serious situation; the total discharge port of the sewage treatment station uses an open channel flowmeter, which may cause an error in the total discharge water flow rate.

2. Affecting the environment

  After a large amount of bio-bubble is generated, it spreads to the aisle plate and affects normal maintenance. The bio-foam may freeze in winter and it is difficult to clean up; in the summer, it will wind up, forming a bad smell and seriously polluting the environment.

3．Dissolved oxygen reduction

  In the process of surface aeration equipment, the bio-foam has viscosity, which prevents normal aeration and oxygenation, and reduces the dissolved oxygen of the mixture.

4 The effluent water quality is worse

  Some bio-foams may also enter the secondary settling tank, causing an increase in pollutants such as SS and CODcr in the external drainage.

Foam control method

1.   Spraying water

  The high-speed spraying of water or water droplets can break up the bubbles floating on the surface of the water, and the partially dispersed sludge particles can restore the sedimentation performance and reduce the foam. By spraying water, the foam can be reduced, and if the aerobic pool is sprayed, the long-term defoaming effect can be achieved. Although spray water cannot be defoamed at all, it is the simplest and most common physical method.

2.   Add chemical agents

  Adding chemicals can solve the foam problem in a short time, and the operation is simple. However, the addition of chemical agents will have a great impact on the sludge while solving the foam problem, and the use of chemical agents will have a greater impact on the quality of the effluent and the disposal of the remaining materials.

  Common dosages: 1. Add chlorine and oxidant; 2. Add coagulant; 3. Add defoamer and vegetable oil.

3. Shorten sludge retention time

 Reducing the sludge residence time of the aeration tank, that is, reducing the average residence time of the cells, can effectively control the bio-bubble in the activated sludge process. Reducing the sludge residence time is essentially a biological screening strategy, that is, using the characteristics of the average generation time of the foaming microorganisms, inhibiting the excessive proliferation of the foaming microorganisms in the aeration tank or excluding them, and achieving the control of the biological foam. purpose.

4. Applying the load to the aeration reactor

  Adding mobile or fixed fillers in some activated sludge systems to make some microorganisms that are prone to sludge swelling and foaming grow, which can increase the biomass in the aeration tank, improve the treatment effect, and reduce or control the foam. produce.